The present invention is directed to a method for forming g,raft copolymers from perfluoroolefins and perfluorovinyl ethers having fluorosulfonyl and fluorosulfonate functionality with selected polymers, and the uncrosslinked graft copolymers resulting therefrom.
Howard. U.S. Pat. No. 5,798,417, discloses (perfluorovinyl ether)-grafted polyolefins. The grafting process involves contacting the polymer in the form of a powder or a shaped article with a free-radical initiator and the monomer. The reaction medium is heterogeneous and the polymer is invariably crosslinked during the grafting reaction. The degree of grafting varies with the initiator concentration and the reaction temperature. Up to 17 mol % incorporation of grafted monomer is claimed (up to 9 mol % shown in the examples). The polymers are useful as catalysts and as membranes in electrochemical cells.
U.S. Pat. No. 4,396,727 claims cation exchange membranes having fluorovinyl sulfonic acid monomers grafted onto high molecular weight substrates. Grafting is carried out directly on films by using ionizing radiation. Substrate polymers have the following repeat units: CH2xe2x80x94CXY where X is H, F. or CH3, and Y is H or F. Polyethylene is the most preferred substrate. Solvents miscible with the monomer can be used to achieve thorough impregnation of the substrate. Graft ratios of up to 75% are reported (although graft ratio is not defined, it is commonly found in the grafting literature as (wxe2x88x92w0)/w0 where w0 is the weight of the substrate and w is the weight after grafting. So 75% graft ratio is equivalent to 43 weight % grafting). The membranes are useful in electrochemical cells. U.S. Pat. No. 4,384,941 claims a process for electrolysis of pure water using these membranes.
Drysdale et al., WO 98/31716 discloses free radical grafting of partially fluorinated functionalized vinyl monomers to polyethylene. In the process, the polyethylene is first dissolved and the grafting reaction takes place in solution. Incorporation of up to ca. 13 mol-% is achieved. The polymers are useful as molding resins, for coatings and as catalysts.
DesMarteau, U.S. Pat. No. 5.463,005 discloses fluoromonomers containing sulfonimide groups and their copolymers with tretrafluoroetllylene. Conductive compositions of these materials are also disclosed.
Armand et al., EP 0,850,920 A2, teaches salts of perfluorinated amides and their use as materials for ionic conduction. Polymers containing sulfonimide side groups are disclosed. Examples include several condensation and addition polymers containing hydrocarbon backbones.
Narang et al., U.S. Pat. No. 5,633,098 disclose ionic polymers having sulfonic acid and sulfonimide functional groups. Polymers disclosed include polysiloxanes, polymethacrylates, and poly(alkene oxides).
Considerable interest has developed in the application of fluorinated ionomers as solid polymer electrolyte membranes in secondary lithium batteries and fuel cells. Key to these applications is the use of fluorosulfonates or derivatives thereof as cation exchange groups. It is believed that the cations associated with these functional groups only become sufficiently labile when highly electron-withdrawing fluorines are employed proximate to the sulfoilate and sulfonate derivatives, typically, in groups represented by the formula xe2x80x94CF2CF2SO3H or xe2x80x94CF2CF2SO3Li or sulfonyl imide or sulfonyl methide derivatives thereof. See for example Doyle et al., WO 9820573, Doyle et al., WO 9941292(A1), Feiring et al., WO 9945048(A1).
Polymer having a backbone of methylene groups and pendant groups having the formula xe2x80x94CH2CH2xe2x80x94(CF2)2O(CF2)2SO2F is prepared by a grafting reaction in Choi et al., WO 9952954.
The current state of the commercial art is exemplified by Nafion(copyright) Perfluoroionomer Membranes available from E. I. du Pont de Nemours and Company, Wilmington, Del. Nafion(copyright) membranes were developed for the highly corrosive environment of a chloralkali cell wherein the corrosion resistance of the perfluorinated ionomers is an important attribute. It is believed that in certain other applications such as lithium batteries corrosion resistance may be of less importance. In such a case considerable reduction in materials cost may be achieved by reducing the fluorine content in parts of the molecule which do not affect ionic conductivity. See for example Choi et al., WO 9952954.
The present invention provides a method for combining a perfluorinated functional group with a polymer having a backbone which contains carbon hydrogen bonds with the aim of providing a non-cross-linked, highly processible lower cost ionomer of high ionic conductivity.
The present invention provides for a process comprising:
contacting a first polymer having a backbone which comprises at least 50% methylene units with a solvent which swells or dissolves said first polymer to form a solvent-swollen polymer or polymer solution;
contacting said solvent swollen polymer or polymer solution with a source of free-radicals and a compound of the formula F2Cxe2x95x90CFR1R2SO2X wherein
R1 represents a covalent bond or a perfluoroalkenyl radical having 1 to 20 carbon atoms; R2 is a radical of the formula:
xe2x80x94Oxe2x80x94[CF2CF(R3)xe2x80x94Om]nxe2x80x94CF2CF2xe2x80x94
wherein m=0 or 1, n=0,1, or 2, and R3 is F or a perfluoroalkyl radical having 1-10 carbons; and X is F or the radical represented by the formula
xe2x80x94Y(M)(SO2R4)p
wherein Y is C or N, M is an alkali metal, R4 is a perfluoroalkyl radical having 1-10 carbons optionally substituted with one or more ether oxygens, and p=1 or 2 with the proviso that p=1 when Y is N and p=2 when Y is C;
to form a reaction mixture;
providing sufficient heat to said reaction mixture to cause the initiation of free-radical reaction; and, reacting said mixture to form a graft copolymer.
The present invention further provides for a non-crosslinked polymer comprising a polymer having a backbone which comprises at least 50 mol-% methylene units and up to 50 mol-% of methylene units having a pendant group comprising a radical represented by the formula 
wherein R1 represents a covalent bond or a perfluoroalkenyl radical having 1 to 20 carbon atoms; R2 is a radical of the formula:
xe2x80x94Oxe2x80x94[CF2CF(R3)xe2x80x94Om]nxe2x80x94CF2CF2xe2x80x94
wherein m=0 or 1, n=0, 1, or 2, and R3 is F or a perfluoroalkyl radical having 1-10 carbons, and X is F, xe2x80x94OM, or the radical represented by the formula
xe2x80x94Y(M)(SO2R4)p
xe2x80x83wherein Y is C or N, M is hydrogen or an alkali metal, R4 is a perfluoroalkyl radical having 1-10 carbons optionally substituted by one or more ether oxygens, and p=1 or 2 with the proviso that p=1 when Y is N and p=2 when Y is C.
The present invention provides a process for preparing non-crosslinked graft copolymers by reacting a perfluoro vinyl compound comprising a sulfonyl fluoride functionality or derivative thereof with a polymer in the presence of a free radical initiator and a suitable solvent.
As used herein, the term xe2x80x9creactingxe2x80x9d is intended to mean allowing at least two components in a reaction mixture to react to form at least one product. xe2x80x9cReactingxe2x80x9d may optionally include stirring and/or heating or cooling.
In the process of the invention, the polymer is contacted with a solvent, which swells or, preferably, dissolves the polymer. The swollen polymer or polymer solution is further contacted with a perfluorovinyl compound comprising a sulfonyl fluoride functionality or derivative thereof and a source of free radicals. Preferably the perfluorovinyl compound is soluble in the solvent which swells or dissolves the polymer.
Polymers suitable for the process of the invention are those which have hydrogens along the backbone which can be abstracted by a free radical initiator. Suitable are polymers having at least 50 mol-% of methylene units in the polymer backbone. Most olefinic type polymers are suitable; suitable olefinic type polymers may be fluorinated but not perfluorinated. Preferred are polyethylene polypropylene, and their copolymers, as well as copolymers of ethylene with acrylates, methacrylates, and vinyl acetate terpolymers thereof with olefins having 3 or more carbons, and combinations thereof. Polyethylene is the most preferred.
Preferably, the source of free radicals is a free-radical initiator such as is well known in the art. Free radical initiators suitable for the process of the invention include inorganic peroxides and organic peroxides and azo compounds. Organic peroxides are preferred; tert-butyl peroxide and dicumyl peroxide are most preferred. The amount of initiator used is between 1 and 20 weight % of the polymer, preferably from 5 to 10 weight %.
Fluorinated vinyl compounds containing a fluorosulfonyl fluoride functional group or a derivative thereof are suitable for the practice of the invention. Suitable for the process of the invention arc perfluorovinyl and perfluoroallyl ethers represented by the formula F2Cxe2x95x90CRF1R2SO2X wherein R1 represents a covalent bond or a perfluoroalkenyl radical having 1 to 20 carbon atoms; R2 is a radical of the formula:
xe2x80x94Oxe2x80x94[CF2CF(R3)xe2x80x94Om]nxe2x80x94CF2CF2xe2x80x94
wherein m=0 or 1, n=0, 1, or 2, and R3 is F or a pertluoroalkyl radical having 1-10 carbons; and X is F or the radical represented by the formula
xe2x80x94Y(M)(SO2R4)p
wherein Y is C or N, M is an alkali metal, R4 is a perfluoroalkyl radical having 1-10 carbons optionally substituted with one or more ether oxygens, and p=1 or 2 with the proviso that p=1 when Y is N and p=2 when Y is C.
Preferably R1 is a bond or xe2x80x94CF2xe2x80x94, R3 is a perfluoroalkyl radical having 1-4 carbons, m=1, n=0 or 1, and X is F. Most preferably R1 is a bond, R3 is xe2x80x94CF3 and n=1.
The grafting reaction is carried out by contacting the perfluorovinyl compound, which is usually a liquid at the reaction temperature, with the polymer in the presence of the solvent. The polymer can be dissolved in a solvent or just swollen by the solvent. Higher incorporation of the monomers is achieved when the polymer is completely dissolved in the solvent. Suitable solvents include aromatic hydrocarbons such as chlorobenzene and dichlorobenzene, halogenated hydrocarbons, and polar aprotic solvents such as dimethyl acetamide and dimethyl formamide. Chlorobenzene is preferred.
In the preferred embodiment of the invention the grafting reaction is carried out by first dissolving or swelling the polyethylene in chlorobenzene at the reaction temperature, adding a fluorinated vinyl compound, preferably perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PSEPVE) and free-radical initiator, and stirring the mixture under an inert atmosphere. The concentration of chliorobenzene can range from about 30% by weight in the swollen polymer to about 99% in a polymer solutions. Most preferred are solutions having polyethylene concentrations of 3 to 6% by weight. Reaction temperatures can be typically between room temperature and about 160xc2x0 C. depending on the solubility of the polymer and the decomposition temperature of the initiator. The most preferred range is between 80 and 130xc2x0 C. An inert atmosphere is necessary to avoid reaction of the free radicals with oxygen. Nitrogen and argon are suitable atmospheres.
According to the present invention, grafting occurs at the methylene units along the polymer backbone. However, steric considerations prevent grafting on nearby methylene units. Thus, in the preferred embodiment wherein the backbone consists entirely of methylene units, a maximum degree of grafting is achieved with slightly more than 20% of methylene units being subject to grafting. It is found in the practice of the invention that the graft polymers obtained in the preferred embodiment of the process of the invention contain from 10 to 87 weight percent of the radical derived from the PSEPVE. The degree of grafting depends on the temperature and on the concentration of polymer, fluorinated vinyl compound, and initiator, as shown in the examples below.
The graft polymers of the present invention not being crosslinked exhibit excellent processibility. The non-ionic sulfonyl fluoride forms are melt processible by conventional means such as extrusion or molding, and are soluble in fluorinated solvents, aromatic hydrocarbons such as chlorobenzene, dichlorobenzene, toluene, and xylene, and halogenated hydrocarbons such as tetrachloroethane. The ionic forms, such as the imides, methides, sulfonates, and sulfonic acids, of highly grafted low MW polymers are soluble in water and methanol. This is in contrast to the graft polymers of Howard, op.cit., which are crosslinked and therefore insoluble.
The polymer produced by the process of the present invention is also more uniformly grafted than in Howard. In the process of Howard, grafting occurs predominantly at surfaces of particles and shaped articles. In the most preferred embodiment of the present invention, wherein the reaction is performed in solution, mixing of the reactants takes place on the molecular scale, affording a considerable improvement in homogeneity.
In another embodiment, the process of the invention may be conducted by swelling porous particles of polymer in a mixture of solvent and fluorinated vinyl compound also containing free-radical initiator, and heating them below the melting point of the polymer. As in the preferred embodiment hereinabove described, the degree of grafting depends on the ratio of fluorinated vinyl compound to solvent as shown in the examples below. Suitable fluorinated vinyl compounds, solvents, and initiators are the same as hereinabove described above for the preferred embodiment. In the present embodiment, reaction temperatures should be kept below the lower of polymer melting point or dissolution temperature of the polymer in the reaction mixture.
The graft copolymers of the present invention can be processed into films by known melt-processing methods as well as by solution casting methods. Tough transparent films are obtained by both methods.
The successful practice of the process of the invention depends upon the dispersibility of the fluorinated vinyl compound within the solvent-swollen or dissolved polymer. It is found in the practice of the invention that the best results are obtained when the fluorinated vinyl compound is itself soluble in the solvent. When the fluorinated vinyl compound is not soluble in the solvent, obtaining a homogeneous dispersion of the fluorinated vinyl compound in the polymer is much more problematical. For this reason, it is preferred to perform the grafting reaction with the fluorinated vinyl compound in the form of a sulfonyl fluoride, followed by hydrolysis to the sulfonate form or imidization or methidization to the respective imide or methide form, as shown in the specific embodiments provided. The sulfonate form can further be converted to the sulfonic acid by acid exchange.
In a preferred embodiment, polyethylene is dissolved as described hereinabove, and the resulting solution combined under the above conditions with a free radical initiator and a fluorinated vinyl compound represented by the formula F2Cxe2x95x90CFR1R2SO2X wherein R1 is a perfluoroalkyl radical or a covalent bond, R2 is a radical of the formula:
xe2x80x94Oxe2x80x94[CF2CF(R3)xe2x80x94Om]nxe2x80x94CF2CF2xe2x80x94
wherein m=0 or 1, n=0, 1 or 2, and R3 is F or a perfluoroalkyl radical having 1-10 carbons; and X is F. More preferably R1 is a perfluoroalkyl radical having 1-4 carbons or a covalent bond, m=1, n=0 or 1, and R3 is a perfluoroalkyl radical having 1-4 carbons. Most preferably R1 is a covalent bond, and R3 is xe2x80x94CF3. The product so formed is the related graft copolymers in sulfonyl fluoride form.
The sulfonyl fluoride polymer obtained according to the preferred process of the invention can be transformed into ionomers by hydrolysis procedures which are known in the art; see for example Doyle et al., WO 9941292(A1). For example, the sulfonyl fluoride groups can be transformed to sulfonates by reacting with alkali metal hydroxide solutions such as LiOH in water, water/methanol, and water/DMSO. Hydrolysis temperature ranges from room temperature to about 90xc2x0 C.
While it is within the scope of the process of the invention to combine an unsaturated disulfonyl imide or methide perfluorovinyl ether, such as described in Feiring et al., WO 9945048(A1) with the polymer, and perform the grafting reaction as hereinabove described, the limited solubility of the imide in ordinary solvents provides an incentive to perform the imidization, or methidization step, after the graft copolymer is formed in the sulfonyl fluoride form.
In a preferred embodiment, in a second step, a perfluorosulfonamide is reacted with the sulfonyl fluoride form of the graft copolymer in the presence of a base. Isolation of the polymer is not necessary, and is less preferred, and the second step can be performed immediately after grafting. High conversions are achieved. Triethylamine is the preferred base but other bases soluble in the reaction media can be used.
In a more preferred embodiment, triethylamine is used as the base in the second step of the reaction. Surprisingly, the imidized polymer so formed, obtained as the triethylammonium imide salt, is soluble and tractable, allowing films to be prepared either by casting from solution or by melt processing. Transformation to the lithium salt is easily achieved by reacting with lithium hydroxide solutions. Reactive solutions include LiOH in water, water/methanol, and water/DMSO. Hydrolysis temperature ranges from room temperature to about 90xc2x0 C. The polymer is less soluble in the lithiated form.
In a further embodiment of the process of the invention the polymer can be isolated after the grafting reaction, and the second step carried out as described above on the purified polymer. The results are the same. By this method other solvents and bases, that are not compatible with the grafting reaction, can be used.
The non-ionic sulfonyl fluoride form of the graft polymers obtained by the process of the invention are not crosslinked and therefore soluble in numerous organic solvents including aromatic hydrocarbons such as chlorobenzene, dichlorobenzene, toluene, and xylene, and halogenated hydrocarbons such as tetrachloroethane. The ionic form of the graft polymers obtained by the process of the invention are largely insoluble in most organic solvents, but are of limited solubility in water and alcohols. However, it is observed in the practice of the invention that when the triethylammonium imide is formed according to the process of the invention, the product stays in the reaction mixture. It is not known whether the mixture is a true solution or a fine dispersion.
Any method known in the art for converting a small molecule having sulfonyl fluoride functionality to imide or methide has been found to be suitable for the practice of the process of the present invention for converting the sulfonyl fluoride form of the graft copolymers of the present invention. Various methods are described in Des Marteau et al., Inorg. Chem., 1984, 23, 3720, Des Marteau et al., Inorg. Chem., 1990, 29, 2982, Canadian patent 2000142-A; U.S. Pat. No. 4,505,997; and U.S. Pat. No. 5,072,040. Preferably, the sulfonyl fluoride is contacted with a perfluorosulfonamide, preferably perfluoromethylsulfonylamide, in the presence of triethylamine. A description of the preferred method is found in Hamrock et al., WO 99/49529.
In a further embodiment, it is found that uncrosslinked sulfonyl fluoride graft copolymers prepared according to the method of Drysdale et al., op cit., are also converted at high yield according to the same imidization procedures found to be effective for the sulfonyl fluoride form of the graft copolymers of the present invention. In a preferred embodiment, a polymer having methylene monomer units and a pendant group having the formula
xe2x80x94CH2xe2x80x94CH2xe2x80x94CR5R6R7X
wherein R5 and R6 are each independently fluorine or perfluoroalkyl, R7 is fluorinated alkylene optionally containing ether oxygen and X is xe2x80x94SO2F, is contacted with perfluorosulfonamide, preferably perfluoromethylsulfonylamide, in the presence of triethylamine to form the associated imide which may then be readily ion exchanged to form the lithium ionomer useful in batteries.
When forming the ionic species of the invention, namely the sulfonates, sulfonic acids, imides and methides as herein described, the methods employed in the art for small molecules rather than polymers may be employed. However, solubility considerations make the reactions more problematical. The solvents typically used for these reactions with small molecules are polar aprotic solvents such as acetonitrile, THF, diethyl ether, DMSO. Most of these do not appreciably dissolve the sulfonyl fluoride form of the graft copolymers of the invention, and therefore are less preferred. Preferred instead are chlorobenzene, dichlorobenzene and toluene.
Stability is a further concern in the particular case wherein the polymer backbone comprises monomer units of vinylidene fluoride. Polyvinylidene fluoride is known to be unstable to bases, the methods of the art for forming imides when applied to a graft copolymer of polyvinylidene fluoride result in degradation of the backbone chain.
Once conversion to the imide or methide form is accomplished e.g., to form the triethylammonium imide or methide, the lithium imide or methide may be formed according to ordinary ion exchange means as is well known and widely practiced in the art, and as hereinabove described.
Ionomers prepared by this process are useful for electrochemical applications such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, or electrodes.